1. Field of the Invention
This invention relates to a glass for an element of a Faraday rotator which has a large Verdet constant, and a reduced absorption of light in the visible and infrared light wavelength regions and which is stable as glass.
2. Description of the Prior Art
It is well known that glass containing large quantities of rare earth ions which exhibit paramagnetism has a large Verdet constant and is used as an element of a Faraday rotator. In particular, glass containing a Tb.sup.3.sup.+ ion has a large Verdet constant and a reduced absorption of light in the visible and infrared wavelength regions, and the terbium ion in a high temperature glass solution retains its stable trivalent state. Accordingly, it is possible to obtain glass for a Faraday rotator element having a large Verdet constant and a reduced absorption of light in the above-mentioned regions by a simple melting method without the need to resort to any special melting method.
In order to increase the Verdet constant, the concentration of the Tb.sup.3.sup.+ ion in the glass should be increased. Heretofore, a borate-type glass capable of containing a large quantity of the Tb.sup.3.sup.+ ion has been used as a matrix glass. Increasing the Tb.sup.3.sup.+ ion results in poor stability of glass same as in the case of other rare earth ions, and it becomes difficult to obtain homogeneous glass of a large size. Attempts have therefore been made to improve the stability of the glass by adding a small amount of Al.sub.2 O.sub.3, ZnO or SiO.sub.2, etc. However, when a large quantity of Tb.sub.2 O.sub.3 is incorporated, conventional glasses cannot be expected to have such stability as to give homogenous and large-sized products.